The invention relates to a screw conveyor or a screw press for conveying compressible or compactable material through a tubular elongated casing from the inlet end to the outlet end thereof. The material collects in the grooves between the flights of the helix and is advanced through the bore of the casing by the rotation of the screw while being compressed progressively during its passage from the inlet end to the outlet end where it acquires the form of a substantially compact plug.
The invention is particularly applicable to a defibrating apparatus of the well-known Asplund Defibrator type, as shown, for example, in U.S. Pat. No. 2,145,851. In such apparatuses, the ligno-cellulosic material, such as wood chips, shavings, sawdust and bagasse, etc., are fed from a hopper or storage bin into a casing which houses a helical screw feeder which transports the material to further pulp processing equipment, which may include mechanical defibrators or other disc refiners, digesters and other pressure vessels, wherein the material is maintain in an atmosphere of pressurized steam. In addition to conveying the pump material from the hopper to the subsequent pulp processing station, the screw feeder also serves to progressively compress or compact the fibroius material during its passage through the bore of the casing, so that it forms a plug at the discharge end thereof, which acts as a seal against blow-back of steam or other medium from the pressurized system.
When employing conventional screw feeders, it has been shown that friction created by the compacting process often causes the material to be rotated along with the helix, and thus interrupts the uniform flow of material through the apparatus, with consequent plugging problems and costly machinery downtime in the system.
These problems have been partly solved by rifling the bore of the casing or the feeder throat, as it is commonly referred to in the pulp manufacturing industry. It has been shown, however, that material such as sawdust, shavings, straw, bagasse, waste paper and semi-cooked chips do not possess sufficient internal friction or consistency to be arrested by the rifling and will still rotate along with the feed screw. Thus, the material is merely subjected to a churning action, with consequent minimized efficiency of the system or complete work stoppage.